1. Field of the Invention
The present invention relates to a shaft supporting structure for an axial fan.
2. Description of the Related Art
A typical heat-dissipating fan includes a fan mounted in a casing having an axle tube. A bearing means is force-fitted into the axle tube, a central shaft of the fan wheel (rotor) is extended through the bearing, and a retainer (e.g., a C-clip) is mounted in an annular groove in a distal end of the fan wheel, thereby rotatably mounting the central shaft of fan wheel in the axle tube on the casing. The bearing means may include a single bearing or two bearings. For example, Taiwan Utility Model Publication No. 235696 issued on Dec. 1, 1994 and entitled "IMPROVED SELF-LUBRICATING BEARING FOR COOLING FANS", as shown in FIG. 6 of the drawings, discloses a single bearing type structure, wherein the bearing 30 is engaged with an overall area of an inner periphery of the axle tube 23 for rotatably supporting the central shaft 14. Nevertheless, the assembly procedure is troublesome and wear is severe since the contact area between the central shaft and the bearing is increased. Longevity of the fan is thus shortened. Another type is disclosed in Taiwan Utility Model Publication No. 287588 issued on Oct. 1, 1996 and entitled "INTEGRAL CASING/AXLE TUBE FOR HEAT-DISSIPATING FANS", as shown in FIG. 7 of the drawings, which discloses a dual-bearing type structure, wherein two bearings 77, 78 are respectively mounted to two ends of the axle tube 76 by means of force-fitting to reduce contact area in a mediate portion of the axle tube 76. Nevertheless, a slight deviation or force-fitting difference during mounting of the bearings 77, 78 will result in misalignment in the central axis of the bearings 77, 78 and the central axis of the central shaft 91 of the fan wheel 90. As a result, rotation of the central shaft about a fixed axis cannot be guaranteed, and noise tends to occur.